Decelerating and scan expansion lens system for electron discharge tube incorporating a microchannel plate

Abstract

An electrostatic decelerating and scan expansion lens system (10) includes a mesh element (56) and operates in a cathode-ray tube (12) that incorporates a microchannel plate (24). The lens system is positioned downstream of the deflection structure (42 and 44) and provides linear magnification of the electron beam deflection angle. The mesh element is formed in the shape of a convex surface as viewed in the direction of travel of the electron beam (40) to provide a field with equipotential surfaces (100) of decreasing potential in the direction of electron beam travel. Secondary emission electrons generated by the mesh element as it intercepts the electron beam, are therefore, directed back toward the lens system and not toward the microchannel plate. Only the beam electrons strike the microchannel plate, which provides on the phosphorescent display (20) an image of high brightness, free from spurious light patterns.

We claim: 
     
       1. A deceleration and scan expansion electron lens positioned between deflection structure and a target structure of an electron discharge tube, comprising: a tubular electrode structure which receives an electron beam exiting the deflection structure and through which the electron beam travels toward the target structure;   a mesh electrode structure positioned to intercept the electron beam after it passes through the tubular electrode structure, the mesh electrode structure including a mesh element formed in the shape of a convex surface as viewed in the direction of travel of the electron beam; and   means for applying a bias potential between the tubular electrode structure and the mesh electrode structure, the mesh electrode structure having a negative potential relative to that of the tubular electrode structure, thereby to expand the deflection provided by the deflection structure and decelerate the beam electrons as they travel through the tubular electrode structure toward the target structure.   
     
     
       2. The electron lens of claim 1 in which the mesh element is of rotationally symmetric shape. 
     
     
       3. A cathode-ray tube, comprising: means for producing a beam of electrons directed along a beam axis in the tube toward a remote display screen;   deflection means for deflecting the beam relative to the beam axis;   electron multiplying means positioned adjacent the screen to receive the electron beam and provide an increased number of electrons to the display screen and thereby enhance display image brightness; and   a decelerating and scan expansion electron lens positioned downstream of the deflection means and upstream of the electron multiplying means to magnify the amount of electron beam deflection produced by the deflection means and to decelerate the electrons in the deflected electron beam to prevent the movement of secondary emission electrons toward the electron multiplying means and thereby prevent the production of spurious light image patterns on the screen caused by such electrons.   
     
     
       4. The tube of claim 3 in which the electron lens develops an electric field through which the beam of electrons travels and comprises a mesh element formed in the shape of a convex surface as viewed in the direction of travel of the beam of electrons. 
     
     
       5. The tube of claim 4 in which the electron lens develops a first electric field and in which there exists a region within the tube between the electron multiplying means and the electron lens, the region including a second electric field of substantially lower intensity than that of the first electric field. 
     
     
       6. The system of claim 5 in which the first electric field produces lines of force having axial components projected onto the beam axis in the direction opposite to that of the direction of travel of the beam of electrons to prevent the attraction of secondary emission electrons dislodged from the mesh element toward the screen. 
     
     
       7. The system of claim 4 in which the mesh element is of rotationally symmetric shape. 
     
     
       8. The system of claim 3 in which the electron multiplying means comprises a microchannel plate. 
     
     
       9. In an electron discharge tube having an electron gun positioned at one end of the tube for producing a beam of electrons directed along a beam axis in the tube and deflection means for deflecting the electron beam to form an image, an electrostatic lens system positioned downstream of the deflection means along the beam axis and comprising: a decelerating and scan expansion lens including a first electrode structure and a mesh electrode structure supported downstream of the first electrode structure, the first electrode structure and the mesh electrode structure cooperating to develop an electric field through which the beam of electrons travels, the electric field being of a character that linearly expands the electron beam deflection provided by the deflection structure and decelerates the beam electrons as they propagate through the electric field; and   a target structure having an input member to which a potential is applied to produce an electric field of relatively low intensity that attracts the beam electrons but not secondary emission electrons dislodged from the mesh electrode.   
     
     
       10. The tube of claim 9 in which the first electrode structure comprises a first tubular electrode through which the beam of electrons propagates. 
     
     
       11. The tube of claim 10 in which the mesh electrode structure comprises a mesh element that is formed in the shape of a convex surface as viewed in the direction of movement of the beam of electrons and forms electric field lines that are contained substantially within the first tubular electrode. 
     
     
       12. The tube of claim 10 in which the mesh electrode structure comprises a second tubular electrode that is coaxially aligned with and overlaps a portion of the first tubular electrode by an amount that provides for corrected geometry of the image. 
     
     
       13. The tube of claim 12 in which each of the first and second tubular electrodes is of cylindrical shape. 
     
     
       14. The tube of claim 9 in which the input member of the target structure comprises an electron multiplier that increases the number of electrons striking the screen and thereby provides an image with high brightness. 
     
     
       15. The tube of claim 14 in which the electron multiplier comprises a microchannel plate. 
     
     
       16. A cathode-ray tube, comprising: an image display screen comprising a layer of phosphorescent material;   an electron multiplier positioned adjacent the screen and including input means for receiving a beam of electrons and output means for providing an increased number of electrons to the screen;   means for producing a beam of electrons directed along an axis toward the input means of the electron multiplier;   deflection means for deflecting the beam away from the axis; and   a divergent electron lens disposed intermediate the deflection means and the electron multiplier for increasing the amount of electron beam deflection produced by the deflection means, the lens including means for providing a decelerating electric field between the deflection means and the electron multiplier.   
     
     
       17. The cathode-ray tube of claim 16 in which the electron lens comprises a conductive mesh element disposed in the path of the beam. 
     
     
       18. The cathode-ray tube of claim 17 in which the electron lens comprises a first tubular electrode disposed in alignment with the axis and a second tubular electrode aligned coaxially with the first electrode, the second tubular electrode supporting the mesh element at one end thereof. 
     
     
       19. The cathode-ray tube of claim 18 in which the mesh element is maintained at a negative potential relative to that of the first tubular electrode. 
     
     
       20. The cathode-ray tube of claim 16 in which the electron multiplier comprises a microchannel plate.